The first gas laser ever built and demonstrated, the helium-neon laser, made use of a relatively long glass tube to contain the sub-atmospheric pressure mixture of gases that made up the active medium, and to confine the electric discharge that excited the gas mix. This design approach was used as well in the early development of the carbon dioxide (CO.sub.2) laser. In the first CO.sub.2 laser, built by C. K. N. Patel in 1964, the electric discharge was in a glass tube off to the side of the main tube. This discharge excited nitrogen that was then mixed with CO.sub.2 in the main tube. It was soon realized that the more simple configuration employing an electric discharge through a homogeneous mixture of the gases in the main glass tube gave the best performance. Advantages for the glass tube approach include:
1. The inherent simplicity and economy that comes from maintaining a coaxial arrangement of the optical, gas flow, and electric current axes. The gas and the electric discharge are thus confined to the same volume--always desirable for maximum efficiency.
2. Easy availability of glass tubing in a wide range of sizes, and easy fabrication of one-piece structures that typically includes co-axial liquid cooling jackets and various connection ports.
3. Design geometry that provides favorable conditions for generation of an output beam with circular symmetry. With this approach all fluid, heat, and electric current flow is basically symmetrical with respect to the optical axis.
Other design concepts arrange the optical, gas flow and excitation axes so they are not all coaxial. Typically one, two or all of the axes are at right angles to each other and glass tubing is not necessarily a significant part of the structure. These designs allow more rapid convective heat transfer out of the active region and for this reason can produce higher output powers with shorter active lengths, but they are more costly to make and typically exhibit output modes that are not circularly symmetric and are of poor quality. For these reasons the conventional coaxial glass tube design approach remains an important one, especially for CO.sub.2 lasers with continuous output powers of less than 1000 watts.
Even with all of the favorable symmetry conditions and despite the best efforts of laser engineers, the goal of achieving a fundamental mode or gaussian (TEMoo) power distribution across the output beam from CO.sub.2 lasers at power levels over 200 watts has remainded an elusive one. The design process proceeds generally as follows:
1 An active length that will provide the desired power output is chosen. Seventy-five (75) watts/meter remains a good figure to use in calculating the active length needed.
2. The radii of curvature for the end mirrors is selected so that the optical cavity is stable, i.e., light rays travelling near and nearly parallel to the cavity axis remains in the cavity after an arbitrarily large number of reflections from the end mirrors. Also, the mirror curvatures are selected to make the mode sizes at the mirrors as nearly equal as possible.
3. With the cavity length (active length plus any extra space needed between the mirrors) and the mirror radii known, computer-generated data is consulted and the limiting aperture for the cavity that allows the TEMoo mode to oscillate with as little loss as possible is selected but still provides enough loss to prevent the TEM.sub.01 and higher order modes from oscillating. One such source is H. Vogelnik and Ti Li, Proc. IEEE, Vol. 54, p. 1312-1329, October 1966. At its best this diameter selection process is not an exact science, and it is most successfully done from the perspective of experience.
For lasers of less than 20 watts output this method works fairly well. For lasers less than 100 watts it works less successfully, and for lasers over 100 watts it doesn't work at all. Lasers designed in the foregoing manner put out all the power expected but not with a TEMoo mode. If a smaller bore diameter is used, the laser still puts out the same poor mode. Several such iterations will finally yield a laser with a much smaller bore diameter that puts out less power than expected and a mode that is still far from perfect. The theoretical optical loss for this laser would show that not even the fundamental TEMoo mode should lase, let alone any of the higher order modes that are evident in the output.
It has been concluded that the plasma tube must be confining and guiding the beam with very little loss, e.g., acting as a waveguide to conduct the beam back and forth between the mirrors. It is well known that light can propagate in waveguide structures made up of either metal or dielectric materials. What has not been obvious is that the losses for waveguide modes are low enough to permit laser oscillation in CO.sub.2 lasers at 10.6 micrometers in glass (pyrex) tubes.
It was recognized that the inner surface of plasma tubes could be reflective for CO.sub.2 laser beams and that this could have a detrimental effect on mode quality. See "10.6 Micron Laser Frequency Control Techniques", by Sasnett et. al., Sylvania Electronics Systems Western Division, Technical Report AFAL-TR-68-210, September, 1968. It was found that by providing periodic changes in tube diameter any reflections are broken up and scattered. This was particularly important in lasers that used dispersive intracavity wavelength selective elements such as a grating or prism. These elements were intended to deflect unwanted wavelengths out of the cavity so oscillation would occur only on the desired wavelength, and it was important that the tube wall did not reflect the unwanted energy in such a way that it could oscillate. Such wavelength selection techniques are only important in lasers of less than about 100 watts.
When a high power CO.sub.2 laser was built, using the aforesaid techniques for calculating the tube diameter, and rings were inserted within the bore tube to break-up and scatter any light that would otherwise have been reflected from the tube walls, the results were disappointing.